1. Field of the Invention
The present invention relates to a catalyst composition used in a hydrotreatment of hydrocarbon oils, and, more particularly, to a highly active hydrotreatment catalyst composition comprising active metals carried in a well-dispersed manner on a carrier which comprises a mixture of zeolite with a specific particle size and a specific particle size distribution and alumina or an alumina-containing material having a specific pore distribution. The present invention also relates to a hydrotreatment process using such a catalyst.
2. Description of the Background Art
Heretofore, catalysts comprising one or more metals belonging to Group VIB or Group VIII of the Periodic Table carried on a refractory oxide carrier have been used for the hydrotreatment of hydrocarbon oils.
Cobalt-molybdenum or nickel-molybdenum catalysts carried on alumina carriers are typical examples of such hydrotreatment catalysts widely used in the industry. They can perform various functions such as desulfurization, denitrification, demetalization, deasphalting, hydrocracking, and the like depending on the intended purposes.
The characteristics demanded of such hydrotreating catalysts are a high activity and the capability of maintaining its activity for a long period of time.
In order to satisfy these requirements, firstly a large amount of active metals should be carried on carriers in a highly dispersed manner and, secondly, the catalyst should be protected from catalyst poisons such as metals, asphalten, sulfur- or nitrogen-containing macro-molecular substances, and the like contained in the hydrocarbon oils.
A measure that has been proposed to achieve the above first object was to provide carriers having a larger specific surface area. A measure proposed to achieve the second object was to control the pore size distribution of the catalyst, i.e., either (i) to provide small size pores through which the catalyst poisons cannot pass or (ii) to provide large size pores with the carrier to increase the diffusibility of the catalytic poisons into the catalyst. These measures have been adopted in practice.
The recent trend of the difficult availability of lighter crude oils in spite of the increased demand of light fractions and high quality oil products increased the demand of hydrotreatment catalysts which have high desulfurization activities and at the same time hydrocracking or denitrification activities. The demand is vital especially in the hydrogenation process of residual oils containing asphalt.
The hydrocracking reaction generally proceeds slower than the hydrodesulfurization reaction, and since both reactions proceed in competition at the same active site, the relative activity ratio of the hydrodesulfurization to hydrocracking reactions remains almost constant in any reaction temperatures, e.g. in a relatively high severity operation purporting a hydrodesulfurization rate of 90%, the cracking rate remains almost constant at a certain level and cannot be increased.
In order to solve this problem a catalyst has been proposed in which acidic compounds, e.g. silica, titania, etc., are incorporated in an attempt of promoting the cracking activity by increasing the amount of acidic sites which can exhibit the cracking activity but not the hydrodesulfurization activity.
When the characteristics of a catalyst is considered, a smaller mean pore size which can provide a larger surface area is advantageous in order to achieve a greater dispersion of active metals throughout the catalyst. Small pores, however, are easily plugged by macro-molecules, metallic components, and the like which are catalyst poisons. A larger pore size, on the other hand, has an advantage of accumulating metals deep inside the pores. Larger pores, however, provide only a small surface area, leading to insufficient dispersion of active metals throughout the catalyst. Thus, the determination of optimum pore size is very difficult from the aspect of the balance between the catalyst activity and the catalyst life.
As mentioned above, when a hydrotreatment involving the cracking reaction is intended, the addition of acidic compounds such as silica or titania is recommended. However, metal oxides which can form acidic sites when mixed with alumina generally exhibit smaller affinity for molybdenum than alumina. Because of this, the addition of a large amount of such acidic compounds lowers the dispersion of molybdenum throughout the catalyst, thus leading to a decreased desulfurization activity of the catalyst.
Furthermore, hydrocarbon oils having a wide boiling range or containing high molecular heavy components, e.g. atmospheric distillation residues (AR), are very difficult to be converted into lighter fractions by hydrocracking even by the addition of metal oxides which are capable of forming acidic sites.
Atmospheric distillation residues (AR) normally contain 50% or more of the fractions which constitute vacuum distillation residues (VR). Such fractions are subjected to the hydrocracking and acidic cracking reactions on molybdenum metal or on acidic sites and progressively are converted into light fractions. The cracking reactions, however, convert such heavy fractions into light gas oil (LGO) fractions with extreme difficulty, and can at most yield fractions equivalent to primary heavy gas oil (VGO) fractions. For example, vacuum distillation residue (VR) fractions can be cracked, for the most part, into a VGO equivalence, but cannot be cracked into lighter fractions. This means that the hydrocracked primary products, i.e. the products once subjected to a hydrocracking reaction, exhibit extremely low reactivity to a further cracking. Thus, it is very difficult to selectively obtain desired light fractions from heavy fractions by using conventional catalysts.
The subject to be solved by the present invention is, therefore, to develop a hydrotreatment catalyst having both high hydrodesulfurization and high cracking activities at the same time. More particularly, the subject involves, firstly, the determination of the optimum mean pore size and the optimum pore size distribution which are sufficient in ensuring high dispersion of active metals, and, secondly, the provision of a large number of acidic sites throughout the catalyst surface without impairing active metal dispersion, thus ensuring further selective hydrocracking of the heavy fractions which are the products of a previous hydrotreatment reaction. A further subject is to provide a hydrotreatment catalyst possessing a longer catalyst life and a higher activity, which ultimately contributes to promoting the economy of hydrocarbon oil processing.